http://www.w3.org/TR/CSS21/visuren.html#z-index
z-index only applies to positioned elements, however, its computed
value is "as specified". Since the computed value can be exposed to
script via getComputedStyle(), I suggest that it would be more useful
if the computed value is "as specified for positioned elements,
otherwise auto" so that scripts get the value actually used.
[Side note: the spec also says "In future levels of CSS, other
properties may introduce stacking contexts, for example 'opacity'."
Does that mean z-index should apply to non-positioned elements
affected by such properties? Hyatt tells me this is already an open
question for CSS3.]
Regards,
Maciej